Generally, infrared rays of a long wavelength band having a wavelength of 2 μm or more are used for human sensors for detecting human bodies, non-contact temperature sensors, gas sensors, and the like, because of its thermal effect and the effect of infrared absorption by a gas. For example, gas sensors can be used for atmospheric environment monitoring and protection, also for early fire detection and the like, and attracts attention in recent years. In particular, there are many absorption bands inherent to individual gases in the wavelength range from 2.5 μm to 6.0 μm, which is a wavelength band suitable for use in gas sensors.
The principle of the gas sensors using infrared rays is as follows. For example, when a gas is injected into the space between an infrared light source and an infrared detecting device, a specific gas absorbs infrared rays having a specific wavelength. Thus, by analyzing the wavelength spectrum before and after the injection of the gas, the type and concentration of the gas can be measured. Here, the infrared detecting devices include, for example, thermal infrared detecting devices such as pyroelectric sensors and thermopiles, and quantum infrared detecting devices using semiconductor light receiving devices. Quantum infrared detecting devices have advantages such as high SNR and high speed response compared with thermal infrared detecting devices.
A quantum infrared detecting device generally forms a PN junction in a semiconductor capable of detecting infrared rays having a wavelength of 2 μm or more, and electrons and holes generated by the absorbed infrared rays in the light receiving layer are converted into electric signals through charge separation by the internal electric field in a depletion layer at the PN junction.
However, the bandgap of a semiconductor capable of absorbing infrared rays having a wavelength of 2 μm or more is as small as 0.62 eV or less. In a semiconductor with such a small band gap, the intrinsic carrier density at room temperature increases due to thermally excited carriers and the electric resistance of devices decreases, making it impossible to obtain sufficient PN diode characteristics. This is because when the intrinsic carrier density is high, leakage current of the device such as diffusion current and dark current increases. Accordingly, infrared detecting devices provided with a cooling mechanism have been conventionally used for quantum infrared detecting devices in order to suppress thermally excited carriers.
Examples of an infrared detecting device that solves such a problem due to the influence of the ambient temperature include a quantum infrared detecting device described in WO2005027228A (PTL 1). The quantum infrared detecting device disclosed in PTL 1 suppresses the diffusion current by the layered structure and the device structure of the compound semiconductor of the sensor portion, and improves the package of signal amplifier ICs and sensors, thereby providing an infrared detecting device which is operable at room temperature and so compact that it was not conventionally available.